In the case of "headphones", the mobile element may be a flat disc or diaphragm made of such material that it can be moved mechanically by adjacent electro-magnetic coils arranged to be energised by a fluctuating electrical current.
In the case of "loudspeakers", the mobile element usually is a cone-shaped element having corrugations extending from its wider end so that, despite the fact that this end of the cone is mounted on a rigid metal frame, the cone as a whole is free to move vibrationally in the direction of its axis. The smaller end of the cone is usually located against transverse movement by a "spider" which also permits free axial movement of the cone. This end of the cone also carries a cylindrical former on which is wound a "voice coil", arranged to float axial in an annular space between magnetic pole pieces. When a fluctuating electrical current is applied to the voice coil, the cone as a whole is driven vibrationally in an axial direction. The cone acts on the surrounding atmosphere to produce pressure waves which create the sensation of sound.
Whatever the form of the mobile element used in a transducer, necessarily as it moves in one direction to produce a pressure wave, it will produce a rarification on the opposite side of the element. This can be considered as the generation of two pressure waves, one 180 degrees out of phase with the other. It will be seen that the two pressure waves tend to interact with one another. One solution to this problem is to provide a baffle plate of sufficient area, and formed with an aperture which is filled by the mobile element. A baffle plate some 8 feet square will effectively supress interference between the two waves. One alternative is to provide some form of reflex enclosure, which has a sound path from the rear of the mobile element to a port directed in the same direction as the front of the mobile element. If that sound path delays the reverse wave by half a cycle, what emits from the port is in phase with the forward wave. There is of course the practical difficulty that the required length of the sound path is different for different frequencies of sound. A third known approach is to provide a sound absorbing or damping enclosure about the rear end of the mobile element. In that arrangement, the reverse wave is absorbed.
The acoustic device of the present invention is clearly distinct from such prior proposals, and does not make use of any baffle, reflex enclosure or sound absorbing or damping enclosure.